fix: Enrollment-Duplikate, IPv6-Adresse, Agent-Status

- Backend: Enroll-Upsert via MAC-Adresse (kein Duplikat bei Neustart)
  HeartbeatInterval auf 30s reduziert
- Go collector: IPv4 bevorzugen, Loopback/Teredo/ISATAP überspringen,
  Link-Local IPv6 ignorieren
- Go main.go: Enrollment nimmt beste IPv4-Schnittstelle statt Networks[0]

Agent/internal/collector/collector.go

@@ -1,6 +1,8 @@
 package collector
 
 import (
+	"net"
+	"strings"
 	"time"
 
 	"github.com/shirou/gopsutil/v3/cpu"
@@ -79,15 +81,41 @@ func Collect() (*Metrics, error) {
 		counterMap[c.Name] = c
 	}
 	for _, iface := range interfaces {
-		if len(iface.Addrs) == 0 {
+		if len(iface.Addrs) == 0 || iface.HardwareAddr == "" {
+			continue
+		}
+		// Loopback und virtuelle Adapter überspringen
+		nameLower := strings.ToLower(iface.Name)
+		if strings.Contains(nameLower, "loopback") || strings.Contains(nameLower, "teredo") ||
+			strings.Contains(nameLower, "isatap") || strings.Contains(nameLower, "6to4") {
 			continue
 		}
 		ni := NetInfo{
 			Name: iface.Name,
 			MAC:  iface.HardwareAddr,
 		}
-		if len(iface.Addrs) > 0 {
-			ni.IPAddress = iface.Addrs[0].Addr
+		// IPv4 bevorzugen, IPv6 als Fallback
+		for _, addr := range iface.Addrs {
+			ip, _, err := net.ParseCIDR(addr.Addr)
+			if err != nil {
+				// Versuche direkte IP-Adresse (ohne CIDR)
+				ip = net.ParseIP(addr.Addr)
+			}
+			if ip == nil || ip.IsLoopback() || ip.IsLinkLocalUnicast() {
+				continue
+			}
+			if ip.To4() != nil {
+				// IPv4 gefunden — nehmen und abbrechen
+				ni.IPAddress = addr.Addr
+				break
+			}
+			if ni.IPAddress == "" {
+				// IPv6 als vorläufiger Fallback
+				ni.IPAddress = addr.Addr
+			}
+		}
+		if ni.IPAddress == "" {
+			continue
 		}
 		if c, ok := counterMap[iface.Name]; ok {
 			ni.BytesSent = c.BytesSent